The present invention relates to a method and apparatus for continuously filtering liquids containing microorganisms, macromolecules and/or fine particles of solid matter as the substances to be filtered out and collecting the substances that are filtered out.
A method and apparatus for filtering liquids containing microorganisms, macromolecules or fine particles of solid matter are disclosed in German Offenlegungsschrift No. 22 54 860. In the method disclosed in this publication, the liquid to be filtered is moved parallel to a filter surface made of a membrane filter having a smooth surface. The static pressure acting perpendicularly on the filter surface is kept lower than a column of water of three meters and a force acts on the particles in the liquid parallel to the filter surface which force is determined by a suitable selection of the flow speed. This force is higher than the contact friction of the particles against the membrane filter caused by the static pressure and maintains a laminar flow at the filter surface. The apparatus for practicing this method includes at least one vessel for the liquid to be filtered, a circular pump connected with that vessel via a pipeline, at least one filter element connected with the pump via a pipeline, and a pipeline leading back from the pump to the vessel. The filter element has a rectangular inner cross section, the direction of flow is horizontal, and the two vertical sides of the rectangle are formed by membrane filters. A drawback of this method and the apparatus used to practice the method, among other things, is that, after filtration, the collected substances are not present as an almost solid cake and cannot be moved from the device as a unit.
In another prior art system, a bag filter, which is designed for high pressure, is disclosed in German Auslegeschrift No. 1,436,296, and includes a rigid, approximately cylindrical, housing having a major axis in approximately the vertical plane, an easily openable cover attached to the housing, a filter bag fastened with its open, lower end to a flange of the housing, and a rigid, liquid-permeable support enveloping the filter bag. The apparatus also includes an inlet line for conducting the liquid to be filtered into the filter bag, an outlet line for conducting the filtrate away from a filtrate chamber, and a device for turning the filter bag inside out for the purpose of cleaning it when a cover is removed. A substantially closed, impermeable hollow membrane body is disposed inside the filter bag and can be connected to a hydraulic pressure medium and blown up to approximately the size of the filter bag. The lower, open end of this hollow membrane body has a removable cover and is connected to a pipe. The filtrate chamber forming an annular chamber between the filter bag and the housing can also be brought into communication with pressurized air or a vacuum through blockable lines. A significant drawback of this device is its discontinuous and complicated mode of operation. If the housing is filled with liquid to be filtered, it is closed and the hollow membrane body increases its volume so that the filtrate is separated from the filtering residue. The residue then fills only a minor portion of the filter bag volume and must be removed from the bag by turning the bag inside out.
In still another prior art system, an apparatus for filtering liquids of the type S 100 is produced by E. Dinglinger KG, Bremen. This apparatus employs a sieve basket which is open at its top and is disposed vertically in a housing. The liquid to be filtered is introduced from the top into the sieve basket, the filtrate is sucked out and the filter residue remains in the sieve basket as sludge. The drawback of this apparatus is that the filter residue of difficultly filtrable media, such as, algae and other microorganisms, is left in the sieve basket as a sludge or mud and not as a solid cake. The surface of the sludge in the sieve basket assumes a paraboloid shape. The removal of the filtered residue generally requires complicated removal of the sieve basket from the housing followed by complicated cleaning of the sieve basket.